Radio receiving circuit



p 1929- H. T. FRIIS ET AL 1,727,010

RADIO RECEIVING C IRCUIT Filed Nov. 20, 1925 Fig.2

Voltage Amplification 1 Wa ve LenqfhMeiers two windings.

Patented Sept. 3, 1929.

UNITED STATES PATENT OFFICE.

HARALD T. FRIIS AND AXEL G. .TEN SEN,

TION 01 NEW YORK.

OF RED BANK, NEW JERSEY, ASSIGNORS TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORA- RADIO RECEIVING CIRCUIT.

Application filed November 20, 1923. Serial No. 675,816.

improved high frequency an'ipliticat ion over a range of wave lengths.

Features of the invention comprise: a high frequency transformer; and,

A manner of associating the transformer with an amplifying system to give improved high frequency amplification over a range of wave lengths.

Radio frequency transformers have been proposed in which an attempt has been made to reduce the losses between primary and secondary windings by dividing the primary and secondary windings each into a number of separated coils and associating each secondarg,- coil in inductive relation with a respective p'imary coil. This form of con strnction requires considerable space and necessitates the use of more wire to obtain the same mutual inductance than can be obtained between a single primary coil and a single secondary coil. Also this mode of construction, comprising as it does, several distinct. units, provides a greater number of parts which have to be duplicated if duplicate transformers are to be built, than in the case where the transformer comprises only able importance in the manufacture on a quantity production basis of transformers having the same characteristics to a high degree of accuracy.

The high frequency transformer of the invention comprises a two-winding transformer of small mechanical dimensions. and of a construction to give the desired mutual inductance with small capacity losses.

The transformer of the invention comprises, preferably, disk-shaped coils, that is, coils of large total diameter and of large depth of winding (measured from the innermost turns to the outermost turns) com- )arcd with the other dimension, which will lX, termed the width of the winding, corresponding to the thickness of the disk. It

This is a matter of considerhas been found that a marked improvement in the general efliciency of the coil for repeating high frequency waves over a considerable wave length range is realized if the total diameter of the coil is kept above a certain minimum value, and if the thickness of the winding and the separation between the primary and secondary coils is kept below a certain maximum value.

1 t has also been found, in accordance with the second mentioned feature of the invention. that the transmission characteristic of the transformer, that is, the relation between the amplitude of the wave transmitted through the transformer and the length of the wave transmitted is greatly influenced by the manner of connecting the transformer in the circuit. With one manner of connecting the transformer, a single-peaked characteristic is obtained, giving high transmission tltCOIt-Zllll wave lengths at the expense of small or practically no transmission at neighboring wave lengths. With the type of connection of the invention, however, the characteristic is less peaked and the transformer exhibits eflicient transmission over a much greater wave length range.

Reference will now be had to the drawing which, in connection with the detailed description 'to follow will afford a more complete understanding of the nature and objects of the invention.

In the drawings: Fig. 1 is a schematic representation of a radio receiving circuit embodying the invention; Fig. 2 is a view partly in section'of a transformer in accordance with the invention, and Fig. 3 shows characteristic curves illustrative of the effect of connecting the transformer into thecircuit in the manner provided by the invention as compared with the effect of any other manner of connection.

In Fig. 1, the usual loop aerial 1 has the center of its inductance grounded and the terminals of the loop are connected across a. tuning condenser 2 and to the radio frequency amplifier A, in the manner disclosed in U. S. Patent No. 1,678,118 granted H. T. Friis, July 24, 1928. That is, the grid of the tube is connected to one loop terminal, and the plate of the tube has connection to the other loop terminal through a small balancmg condenser 3 having a capacity of the same radio frequency amplification may be pro-.

vided as required and may be connected in tandem with amplifier A in the same manner that that amplifier is associated with the amplifier A Following the radio frequency 5 amplifier stages is the detector D, which may be of usual type and preferably has a gridleak resistance and condenser combination 4. The connection to the detector is through a radio frequency transformer T, which may be of the same type as the transformer T. A stage or more of audio-frequency amplification (not shown) may be provided following the detector in accordance with known practice. The receiver R which isillustrated as the usual ear-phones may in practice be any type of receiver, such as a loud speaker.

The filaments are all connected together and to ground, and are, therefore, connected to the center of the loop inductance. This has the effect of impressing on the grid of tube A, only half the voltage developed in the loop, but the advantages resulting from this mode of connection in stabilizin the circuit and sharpening its frequency selectivity more than outweigh the loss in amplitude, as is fully explained in the patent to Friis referred to.

Filament heating current is supplied to all the tube filaments from the source 5, and the space current for all the tubes is supplied from source 6 which may be provided with a tap for supplying a smaller plate voltage to the detector than to the amplifiers.

The operation of the circuit in receiving radio signals will be obvious from the circuit description already given. Loop 1 1s properly oriented and condenser 2 is properly adjusted to render the set selective, as to direction and wave length, of the particular waves to be received. Amplifiers A and amplify the received energy at the radio frequency level and so secure the well known advantages in signal amplification, which is substantially as the square of the radio frequency amplification. After such amplification, the signal is detected at D and is heard in receiver B. To accommodate the set to waves having any one of a wide range of wave lengths, it is essential that transformer T and T transmit any of the waves within this range, and if anything like uniform results are to be secured at the different wave lengths, it is necessary that these transformers give substantially uniform transmission throughout this wave length range.

The construction of the radio frequency transformer of the invention and the manner of its association in the circuit in accordance with the invention to secure uniformity of transmission over a wide Wave length range will now be described.

Referring to Fig. 2, the transformer windings 7 and 8 are illustrated partly in section to show the manner of winding. These coils are disk-shaped and have an inner and outer diameter and a depth of winding (i. e. measured between the inner and the outer diameter) all of which are large compared with the width of winding. In practice these dimensions are conveniently determined by accurately cutting grooves having these dimensions into a block or spool of suitable material such as hard rubber or wood, and winding the coils in the grooves. The outline of the spool is indicated at 9. It is not necessary to construct the transformer in this manner, however, since the windings may be made selfsupporting or may be supported in any desired manner.

It has been found that when coils are wound in this manner and are associated parallel with each other and co-axial with dimensions within certain limits, and with the proper circuit connections, the transmission characteristieof a receiving set such as that indicated in Fig. 1 as measured and plotted between wave length and amplitude of the wave transmitted through the set, will, in general, comprise the well known double-humped curve characteristic of coupled circuits. The primary of the transformer together with the tube A, and its associated circuits forms a. primary of definite inductance and capacity values, and the secondary and the elements of tube A form in like manner, another circuit having its own-characteristics. The transformer furnishes a, coupling between these circuits which consists partly of inductance and partly of'capacity, the latter being due to the capacity between the transformer windings.

It has been found that as the dimensions of these transformer windings and their separation are varied the transmission characteristic of the set varies markedly. By keeping the width of winding very small and increasing the depth a corresponding amount, tlge shunt capacity of each winding is reduced so that the reactance of each winding is more nearly pure inductance. It has been found that as the two coils are brought nearer to each other the two humps of the characteristic are separated farther apart. Thus, in Fig. 3, curve ,A represents a measured characteristic of a particular circuit similar to that shown in Fig. 1. If windings 7 and 8 were moved closer together and another set of readings taken these wouldbe found to define a characteristic with two humps spread farther apart. Conversely, as the windings are spaced apart farlherthe two humps of the char.- acteristic approach each other more closely. i-\cconipanying this eli'ect, there is a definite variation in the central or valley region of the characteristic. This portion and also the humped portions of the curve are controlled to a large extent by the capacity between the windings, which varies not only with their distance of separation but also with their areas. As the capacity between windings changes, it influences both the relative height of the valley portion of the curve with respect to the humped portions and the distance apart of the humped portions. It will be noted. that the area of a winding of given depth increases as the respective inner and outer diameters increase, so that the capacity between windings varies not only with their spacing but also with the coil diameter, assuming constant depth of winding.

Furthermore, the sense of this capacity in the external circuit, whether (depending on the coil connections) the capacity between certain elements of the circuit or between other elements, has a marked effect on the characteristic.

It has been found by experiment that all of the factors which control the shape of the characteristic curve can be reduced to the shape, the mechanical dimensions, the separation between the windings, and the manner o't connection of the coils in the external circuit.-

Taking up first the diameter of the coils, curve A in Fig. 3 is a measured characteris tic of a circuit employing a transformer having a width of winding of 3/64 inch, a separation between windings of 1/16 inch, an inside diameter of 11inch, and a winding depth of 1/2 inch. 1 117 is, found that as the diameter is increased, ,the other dimensions remaining the sam here is a marked increase in the distan between the humps and also a heightening in the valley portion of the curve, for dimensions in the vicinity of 1 inch internal diameter. For example, in the typical set of measurements there was found to be an increase in the distance between the humps of the order of 25% in passing from a 3/4 inch internal diameter to a 1 inch internal diameter coil, whereas in passing from a 1 inch to a 1-1/4inchinternal diameter coil the increase. in distance between the humps of the characteristic was only about one-half this amount, an observed case being about 14%, the other dimensions being the same. There was only a very slight relative heightening in the valley portion of the characteristic in changing from the 3/4 inch internal diameter to the 1 inch internal diaineter coil, while there was 7 a marked heightening of the valley portion in passmg to the 1-1/4 inch internal diameter coil. There is seen, therefore, to be a marked advantage in increasing the internal diameter of the coils in the vicinity of one inch internal diameter, for the same depth of winding.

This effect on the characteristic of increasing the coil diameter for the samedcpth of winding is probably due to the increase in the area of the face of each coil. It is also found that increasing the depth of the windin for the same inner diameter has similar ellects on the characteristic.

Taking up now the eii'ect of the width of the winding, it is found that, other dimensions being the same and being of the order of those stated hereinbefore, a winding width of 1/16 inch gives a characteristic of principally one hump with possibly a minor hump superposed and displaced at a slight distance from the center of the principal hump of the curve, so that as this winding width is reached in the scale of decreasing widths, the coil begins, at least for wave lengths of the order of 200 to 500 meters, to give the broader characteristic due to the development of the two humps. The development of two humps representing the same order of amplificatitm proceeds rapidly as the width is narrowed from 1/16 inch to 3/64 inch, and somewhat less rapidly as the width is changed from 3/64 inch to 1/32 inch. In the region of 1/16 inch to 3/61- inch, therefore, a value for the width of winding is obtained which gives a markedzimprovement in the characteristic in that itrepresents a broader wave length range over which substantially the same order of amplification is obtained.

As pointed out above, the separation between the windings has a marked effect on the characteristic. With the diameter and j width values of the order of magnitude here- 'inbefore stated, itiis found that for greater separation'sthan about 1/4 to 1/2 inch the two humps of the characteristic begin to merge intoone, and that the effect of changing from a separation of 1/4 inch to a separation of..3/16 inch is very marked, at. least .in the wave length region of 200 to 500 meters. Curve B may be referred to, to show the type of curve obtained with a separation of 3/16 inch between windings. Curve A is not directly comparable with curve B since in the case which it represents, two

differences occur in dimensions, namely, the

internal diameter of the coil in the'casc of curve A was 1 inch while, as stated, that in the case of curve B was 1-1/4 inches, and the coil separation in the case of curve A was 1/16 inch, while that in the case of B was 3/16 inch. This separation effect, however, is more strikingly shown by curves A and B than as though they were directly comparable, since the lesser diameter of coil A would, as stated above, tend to bring the humps of the curve closer together than they vould be for a 1-1/4 inch internal diameter, so that in general, the effect on the spread between the peaks of the characteristic will be greater in changing from a separation of 3/16 inch to a separation of 1/16 inch than that shown by comparing curves A and B. It has not been thought necessary or desirable to encumber the drawings with other curves showing the effect of each individual condition discussed.

Curve B represents a desirable characteristic since it shows an amplification which only varies between the extreme limits of about 3.7 and 6.3 (arbitrary units) or about 25% from the mean value over the total range of 260 meters to (300 meters, and within the range of 260 meters to 460 meters, the greatest variation is only about 15% from the mean.

As stated above, it is advantageous to increase the diameter beyond the minimum limits given. Since all of the dimensions above discussed are to an extent dependent upon each other, it follows that if any one dimension is changed very materially there can be somewhat of a relaxation in the limits set for the other dimensions. For example, it is shown above that increasing the diameter of the winding, other dimensions remaining the same, broadens the characteristic. Increasing the width of the winding, on the other hand, gives a narrower characteristic. Hence, if the winding diameter be considerably increased in a given case, the width of the winding can also be increased to some extent without giving any narrower characteristic than that possessed originally in the assumed case. In other words, while the actual limiting dimensions above given are the preferred dimensions, especially in short-wave radio receivers, the invention is not limited to those actual dimensions but only by the relative dimensions, and the invention can therefore be embodied in transformers of much larger actual'. dimensions than are given above, so long as the relative dimensions given are preserved. For examplc, it is found that if the depth of the winding be increased from 1/2 inch to 1 inch, the inner diameter remaining the same, the width of winding can be increased in about the same proportion, and 3/32 of an inch has been found to give the desired form of characteristic with the 1-inch depth of Wind- 111g.

The effect of connecting the coil of the invention into the circuit in the manner in accordance with the. invention is illustrated by curves A and C which represent the characteristics observed with the same coil connected respectively in accordance with the invention (curve A) and -in.the manner which would ordinarily be thought to be the proper manner (curve C). It will be noted that the two manners ofconnection give entirely different shaped characteristics. Curve A results from connecting the plate and grid to respectively opposite. terminals of the two windings, that is, if the inner terminal of the primary is connected to the plate, as indicated in Fig. 1, the outer secondary terminal is connected to the grid. This means, of course, that the terminals which are connected to the filaments (through the necessary batteries) are in the case of one winding the outer, and inthe case of the other winding the inner terminal.-

Connecting the coil in this manner, therefore, gives a much broader characteristic than connecting homologous terminals to the plate and grid, respectively.

This is on the assumption that both windings are made with the same direction of winding. If one of the two windings is turned so as to present its opposite face to the other winding or if the coil is made up by winding the wire in opposite directions in the two coils, then the homologous terminals can be connected to the plate and grid, respective'ly.

It is probable that reversing the direction of winding of one coil and connecting homologous terminals of the coils to the plate and grid elements will result in a characteristic which is not exactly identical with that obtained by the preferred method of winding the coils in the same direction, and connecting respectively non-homologous terminals to the plate and grid elements. This is for the reason that there is a somewhat ditferent distribution of capacity between corresponding portions of the respective windings in the two cases.

It is, of course, more convenient in practiceto wind both coils in the same direction, particularly where they are formed by building up the windings in grooves in a spool or the like.

The reason for the difference in characteristic obtained with the two general types of connection between the coil terminals and the tube elements appears from present information to be that in the one case there is such a relation between the capacity coupling between the transformer windings and the inductive coupling between them as to increase the wave length range throughout which substantially the same amplification is obtained, while in the other case the two kinds of couplings do not cooperate in this manner. This may be due to the phase relations between the electromotive forces set up due to the respective kinds of coupling. Regardless of the particular theory upon which the action depends, the coil and terminal relations determine to a large extent the shape of the characteristic, and proper relations for obtaining a broad characteristic are those above given.

Each of the curves A, B and C was obtained in an amplifier circuit using the socalled peanut type of vacuum tubes. These tubes were in particular the lVestern Electric Company Type N tubes, which have very small grid-to-plate capacity, the plate consisting of a cylinder about 9/ 16 inch long and 5/32 inch internal diameter, and the grid consisting of a spiral forming a cylindrical surface of about 5/8 inch length and 7/64 inch external diameter. The coil which gave the characteristic curve B contained 200 turns of No. 36 Brown and Sharp gauge double silk covered copper wire in each of the windings.

\Vhile each of the curves given in Fig. 3 was taken with transformers having a 1: 1 ratio, it is to be understood that. the invention is not to be limited to transformers having this ratio but that any desired ratio of turns may be employed. In general, if thetransformer is inserted between tubes having different capacities between the. tube elements to which the transformer terminals are connected, it is found advantageous to use a transformer ratio different from unity and to include the greater number of turns in the coil connected to the tube of the smaller capacity.

In Fig. 1 the transformer T is merely indicated in conventional manner, but it is to be understood that this transformer is of the same type of construction as transformer T.

What is claimed is:

1. A radio-frequency transformer comprising only two windings adapted for connection in circuits, each of said windings consisting of a flat disk-shaped coil having an inner diameter of at least one inch and a dimensional ratio of at least 8: 1 between the radial depth of each winding and the width of the winding, said coils being associated parallel and co-axial with each other and separated by a distance at least as small as one half an inch.

2. A radio-frequency transformer comprising only two windings adapted for connection in circuits, each of said windings consisting of a fiat disk-shaped coil having an inner diameter of at least one inch, a radial depth of winding at least as great as half an inch, and a thickness at least as small as one eighth of the depth of the winding, said coils being associated parallel and coaxial with each other and separated by a distance at least as small as one-half an inch.

3. A radio-frequency transformer comprising only two windings adapted for connection in circuits, each of said windings consisting of a flat disk-shaped coil having an inner diameter of at least one inch and a quarter, a radial depth of winding at least as great as half an inch, and a thickness at least as small as one-tenth of the depth of the winding, said coils being associated parallel and coaxial with each other and separated by a distance at least as small as one quarter of an inch.

4. A radio-frequency transformer comprising only two windings adapted for connection-in circuits, each of said windings consisting of a flat disk-shaped coil having an inner diameter of at least one inch and a quarter, a radial depth of winding at least as great as half an inch, and a thickness at least as small as one-eighth of the depth of the Winding, said coils being associated parallel and coaxial with each other and separated by a distance at least as small as one quarter of an inch.

5. A radio-frequency transformer comprising only two windings adapted for connection in circuits, each of said windings consisting of a flat disk-shaped coil having an inner diameter at least as great as the order of one inch, a radial depth of winding at least as reat as the order of half an inch, and a thickness substantially less than one eighth of the depth of the winding, said coils being associated parallel and coaxial with each other and separated by a distance substantially less than one quarter of an inch.

6. A transformer comprising windings in the form of circular coils arranged coaxially, each winding consisting of a single spiral coil conductor having its dimensions in such a fixed relationship to the dimensions of the other and to the distance separating the windings as to give substantially uniform transmission for waves of a wide range of frequencies.

l. A transformer comprising windings coupled together, the dimensions of each winding bearing a predetermined relation ship to the dimensions of the other, and means for maintaining the windings co'- axially related and for so determining the distance separating them with respect to their dimensions that for waves of a wide range of superaudible frequencies the transmission characteristic of the transformer is substantially constant.

I11 witness whereof, we hereunto subscribe our names this 19th day of November A. D.,

HARALD T. FRIIS. AXEL G. JENSEN. 

